Nội dung
- A large-scale study challenged familiar “best practices”
- From “free,” “exclusive” to “today”: power words are no longer a magic boost for opens
- All caps and longer subject lines remain a disadvantage
- The remaining factors: personalization, exclamation marks, and controlled rule-breaking
- AI writing email: don’t let tools automate outdated habits
- What this means for the Vietnamese market
- References
Many marketers are still writing email subject lines using familiar “golden formulas” such as adding emotional words, emphasizing offers, using all caps to grab attention, or keeping them very short. But a new study cited by MarTech shows that many of these rules are no longer as certain as they once seemed, and may even reduce open rates if applied mechanically.
What stands out for Vietnamese marketers is that email today is no longer about “writing by tricks,” but about controlled experimentation. In the context of increasingly crowded inboxes, large-scale data serves as a very practical reminder: what works in one market may not work in another.
A large-scale study challenged familiar “best practices”
MarTech says researchers at the University of Helsinki analyzed 31,812 marketing email subject lines, sent a total of 4.6 billion times. The goal was to test whether copywriting rules repeated for years would really hold up when measured at scale.
The results showed that some common recommendations were not only uncertain but could also backfire. This is notable because many email teams still use subject line templates based on word-of-mouth experience rather than validating them with their own business data.
From “free,” “exclusive” to “today”: power words are no longer a magic boost for opens
The biggest surprise in the study was the group of words often considered “power words,” such as free, exclusive, today, flash, save. According to MarTech, subject lines containing these words actually generated significantly lower open rates than those without them.
A plausible explanation is that these words have become too familiar in email marketing. When recipients see promotional phrases repeated over and over, they can get blurred into the flood of messages in the inbox. In other words, what once created urgency may now be seen as an old formula.
For marketers, the message is not to “ban” these words, but not to assume that adding a few trigger words will automatically make a subject line better. In many cases, direct and natural language can feel more trustworthy.
All caps and longer subject lines remain a disadvantage
The study also found that subject lines containing all-caps words reduced open rates by about 3.3%. That is not too surprising in a context where users are increasingly sensitive to subject lines that feel like they are “shouting” at them in the inbox.
In addition, subject line length also showed a clear correlation with open performance: the longer the line, the lower the open rate, even if the effect of each character was small. The practical takeaway is that brevity still matters, especially when emails must compete in a fast-scrolling mobile environment.
What matters is that short does not mean abrupt. A good subject line still needs enough context for the recipient to understand what the email is about, rather than being short but vague.
The remaining factors: personalization, exclamation marks, and controlled rule-breaking
Not every finding completely overturns older experience. According to the article, personalization delivered a moderate improvement, but not always an outright win. Previous research has produced mixed results depending on the audience and the stage of the buying journey. So personalization should be treated as a variable to test, not an immutable rule.
A single exclamation mark, on the other hand, showed a positive result: subject lines with one exclamation mark increased open rates by nearly 4%. However, this is a case of “less, but in the right place.” Using too much punctuation or overdoing emphasis can still easily make a message feel like spam.
The researchers also noted that some slight deviations from standard writing norms, such as creative punctuation or unusual formatting, were associated with higher open rates. Even so, these differences only work if they remain readable and professional.
AI writing email: don’t let tools automate outdated habits
This is especially relevant for marketing teams using AI to generate email subject lines. MarTech notes that many AI tools are currently asked to produce “high-converting” subject lines based on the old formula: more urgent, more sensational, more promotional.
The problem is that if the input is already based on old “best practices,” AI is simply automating habits that may no longer be effective. A better approach is to have AI generate multiple options, then use A/B testing to check each assumption: short versus long subject lines, personalized versus non-personalized, promotional words versus direct language.
In other words, AI should support experimentation, not replace a marketer’s critical thinking.
What this means for the Vietnamese market
For email marketing in Vietnam, the core lesson is not to “throw out all the old rules,” but to shift from belief in formulas to a habit of testing. Recipient groups in each industry, at each stage of the buying journey, and across each CRM segment will respond very differently to the same subject line style.
Therefore, teams should build a short-cycle email subject line testing process: create multiple variants with AI or copywriters, split them into small send groups, track open rates and post-open metrics, then scale the winning version. This is far more practical than clinging to a list of “open-rate boosting keywords” circulating online.
From a strategic perspective, the most important thing is to balance attention and trust. A curiosity-driven subject line may help get the email opened, but if it does not reflect the content inside, users will quickly lose trust and reduce engagement in later campaigns.
Reference source: MarTech; the study “Do You Want $150 for FREE? Measuring the effect of language on marketing email open rates” published in Ampersand.
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References
- FREE! The email subject line rules that don’t hold up!
- Do You Want $150 for FREE? Measuring the effect of language on marketing email open rates
